Currently, the spas which are offered by manufactures are made with the inner surface of the spa shaped in complicated form at various positions within the spa to create seating for the occupants. Thermal plastic sheeting is used in most cases. It is applied by vacuum forming of the spa internal surfaces against a female mold made of wood and gel-coated fiber glass. The inner surface has also been made in much the same shapes, including seating, by rotational molding. Each spa configuration, forming the pool and the seating shelves which are part of the wall, requires its own particular mold. Manufacturers normally use several molds in order to be able to offer a variety of seating arrangements.
An important feature of spas is the positioning of ports in the wall where jets of water or water/air mix can be located to impinge upon the occupants of the spa. Customarily, once the spa is finished in production, the arrangements of jets is fixed.
Spas can be shaped internally in many different ways. This determines how many people may be seated, how each person may be seated and how impinging jets of air/water aimed. However, manufacturers must limit their variety of models because each one requires its own mold. The molds are large and they are very costly.